In recent years, a multi-carrier method, such as OFDM (Orthogonal Frequency Division Multiplexing), has become the focus of attention as a communication method to realize a fast wireless transmission. This method has merits that it is capable of a multi-path, resistant to fading or the like. However, as transmission signals convolved with a plurality of carriers are added on a time axis, a high peak power is caused, which has disadvantage in power efficiency of a transmission amplifier. Therefore, a clipping processing is occasionally employed to suppress the peak power.
For example, in Patent Document 1, a technology suppressing a ratio of transmission peak power to average power of the multi-carrier signal (termed as “a peak to average power ratio” hereinafter), without lowering a performance of demodulation in a receiving communication apparatus, and a transmission efficiency is disclosed. A multi-carrier signal transmission apparatus as noted in Patent Document 1 comprises:
i) a buffer to keep a multi-carrier signal;
ii) oversampling means oversampling the multi-carrier signal;
iii) detection means detecting a power level of a first threshold value or above in an oversampled multi-carrier signal;
iv) suppression means suppressing the power level of the oversampled multi-carrier signal to be not more than a second threshold value, if the detection means detects the power level of the first threshold value or above;
v) frequency band limiting means removing components outside a band contained in the multi-carrier signal whose power level is suppressed by the suppression means;
vi) replacement means replacing part of sub-carriers in the multi-carrier signal whose components outside the band are removed with the sub-carrier of the multi carrier signal kept in the buffer, followed by providing the resultant multi-carrier signal, in which part of sub-carriers are replaced, to the oversampling means;
vii) transmission means transmitting the oversampled multi-carrier signal if a power level of the first threshold value or above is not detected.
In accordance with the multi-carrier signal transmission apparatus having the above configuration, the replacement means replaces part of sub-carriers in the multi-carrier removed components outside the band with corresponding sub-carriers in the multi-carrier signal, kept in the buffer. Therefore, by a simple means, part of sub-carriers in the multi-carrier signal is prevented from non-linear distortion, which is generated by peak clipping, that is, a suppression of power level.
[Patent Document 1]
Japanese Patent Kokai Publication No. JP-P2005-294996A